If the main and branch wires are different sizes, you need to protect them individually.

For simplicity, I'd just get a spool of 16AWG wire, fuse it at the battery at 15 amps, and use that for main and branches.

 

Why did AEM specify to use 5A fuse for 12VDC power source?

 

You're killing me.

Because the gauge will never draw more than 5A... so you could use 18awg or 20awg wire to power it.

I suggested 16awg so you could add more gauges later if you like.